Fossils present a particular snapshot of an ancient animal, frozen forever in time. In the case of a recently-announced dinosaur fossil, it’s a very specific moment in time: Researchers in North Carolina have discovered what they believe to be a pregnant Tyrannosaurus rex, which has in turn impacted scientists’ ability to determine the sex of certain extinct dinosaurs as well as the study of egg laying in modern birds.

According to a recently published study from North Carolina State University and the North Carolina Museum of Natural Sciences, paleontologist Mary Schweitzer discovered a medullary bone in the femur of a fossilized T. rex from 68 million years ago. Medullary bone, or MB, is found only in female birds, and only before or during the egg laying period. While Schweitzer made the discovery over a decade ago in 2005, the fossil had to undergo testing before they could confirm it. Certain factors, she explains, could fool researchers into thinking there was MB:

All the evidence we had at the time pointed to this tissue being medullary bone, but there are some bone diseases that occur in birds, like osteopetrosis, that can mimic the appearance of medullary bone under the microscope. So to be sure we needed to do chemical analysis of the tissue.

That chemical analysis involved testing for keraten sulfate, which doesn’t exist in other types of bones, and comparing the results to ostrich and chicken bones that had been proven to contain MB. Because of the short window in which it exists, MB is incredibly fleeting; it must be mobilized quickly in order to shell the eggs laid by birds and certain dinosaurs. Schweitzer and her team were lucky that the femur was already broken when they found it. As the press release from NC State News points out, most paleontologists wouldn’t want to break or demineralize a fossil in order to search for tissue so rare as MB.

Confirming the presence of MB leads to two breakthroughs. First, study of the pregnant T. rex will help scientists to chart the evolution of egg laying in dinosaurs’ descendants, modern birds. Second, it allows scientists to actually identify the gender of a dinosaur. As co-author Lindsay Zanno (paleontologist at the North Carolina Museum of Natural Sciences) explains, it’s not as if they didn’t know about mating rituals already—they just weren’t sure who did what:

It’s a dirty secret, but we know next to nothing about sex-linked traits in extinct dinosaurs. Dinosaurs weren’t shy about sexual signaling, all those bells and whistles, horns, crests, and frills, and yet we just haven’t had a reliable way to tell males from females. Just being able to identify a dinosaur definitively as a female opens up a whole new world of possibilities. Now that we can show pregnant dinosaurs have a chemical fingerprint, we need a concerted effort to find more.

And, most excitingly, there’s the prospect of finding more dinosaur DNA. After all, scientists had previously thought that substances like MB couldn’t be preserved over millions of years. Zanno told Discovery News,

Yes, it’s possible. We have some evidence that fragments of DNA may be preserved in dinosaur fossils, but this remains to be tested further.